GM CPI (Central Port Injection) 1992-1995   CSFI (Central Sequential  Fuel  Injection) 1996-2004

4.3 L V6, 5.0 & 5.7L V8 "Vortec".

This page covers just the basics of the CPI and CSFI engines from 1992 through 2004. 


Back in 1992, General Motors introduced a new type of fuel injection system known as  CPI or (Central Port Injection), or also known as CMFI (Central Multi-Point Fuel Injection).  The system was used on 4.3L V6 Vortec engines through 1995, and  underwent a redesign  in 1996 and renamed “Central Sequential Fuel Injection” (CSFI). The system was also added to 5.0L and 5.7L Vortec V8 engines for the 1996 model year.

The first generation  (1992-95) CPI on the 4.3L V6  used one injector called a MAXI injector that is mounted inside the upper part of the intake plenum. From this assembly was six nylon lines with a poppet style injector on each end. It took about 43 PSI to open the poppet injector to spray fuel into the intake port. 

It must be noted on the first generation, all injectors are activated simultaneously when the MAXI injector was energized, three times per crankshaft revolution. Fuel pressure should be at least 54 psi to 60 psi when the key is on, engine off, the operating pressure is around 55-57 psi nominal.  The MAXI injector on/off time is controlled via the pcm by pulse width modulation, which is basically the injector timing and fuel delivery is either shorten or longer base on inputs from the various sensors ( MAP, throttle position, temperature and engine load.  The system is speed density using  a MAP sensor to help calculate fuel delivery based on a pre-determined VE ( Volumetric Efficiency) table in the computer program. There is no MAF sensor on the first generation.

 If anything in the CPI system fails (the MAXI injector shorts out, goes open or leaks, the fuel pressure regulator leaks, any of the lines that connect the MAXI injector to the nozzles leaks or breaks, or any of the individual nozzles leak or become plugged up), the entire assembly has to be replaced as a whole. 

 The Second Generation  CPI was used on the  1996 and up vehicles including the 5.0 and 5.7L V8 until the LS based engines were introduced in 1999, the 4.3L V6 continued with the CSFI. The main improvement is when the MAXI injector in the central housing was replaced with six individual injectors, each of which feeds fuel to its own poppet spray nozzle.  Another noted difference was an addition of a  mass airflow sensor to monitor airflow. Fuel pressure in the CSFI system is around  60 to 66 psi with key on engine off, and the regulator maintains the operating pressure at 55 to 57 psi.

The second generation is also sequential injection, which means each injector is energized individual per cylinder just before the intake valve opens.  This change results in a smooth idle, better throttle response and lower emissions with a slight improvement in fuel economy.  The one good point about this system is if there a injector gets clogged or fails, it can be replaced individually.

There is an  aftermarket replacement system for these applications has been developed that converts the CSFI system into a conventional multi-point fuel injection system.  This eliminates the troublesome poppet style injectors. This modification cannot be used on the 1992-95 CPI.

1992-1995 CPI


The fuel injection assembly                                                                                                               Parts of the 1992-95 CPI system.



The fuel injector looks like a lot of the TBI injectors, with a exception, the bottom shows six separate injector nozzles.


The poppet hoses are attached at each end of the pod with a retainer and tamper proof bolt. 


This is the intake "power valve' use on the 1992-95 4.3L CPI engine, under load this valve opens up allowing extra air flow in the intake. 


1996 and up  CSFI 


The injector pod, showing the proper  position of the fuel regulator vacuum port.              A better view of the injectors in the pod, with retainer plate installed.  4.3L V6 set up shown, the V8 system is similar.


The fuel injector assembly with line and poppet valve.                Breakdown showing the poppet valve on the left. 





To remove the injector from the pod, grasp the injector by the tube fitting, NOT the tube and while pulling gently, push the other side with a dull screwdriver small enough to get between the electrical terminals without damaging them.




Fuel pressure regulator.                              Breakdown of parts: inner o-ring,  filter diaphragm, body  o-ring, spacer and regulator assembly



The distributor is unique, as it does not use a conventional ignition module. The timing is done by a hall effect switch that is triggered by the rotation of a wheel with a slot cut into it. It is synchronized with the #1 cylinder, so the pcm can trigger the #1 injector in the proper phase in conjunction with the CKP ( Crank Position sensor). With the two sensors working together, the injectors and engine timing can be accurately determined. The distributor is driven off the camshaft gear as in a conventional distributor. It is triggered once per revolution or 1X.  Commonly called a CMP (Cam Position Sensor).



When the gap is block on the sensor, it is made active, sending the pulse to the pcm to control injectors and timing. When the path is clear, the signal is off. Along with the CKP, the pcm knows exactly what cylinder is at TDC, and the injector is timed to energized just ahead of the intake valve opening. 

Basic troubleshooting for the HEI-EST ignition system can be found here

Removal and reinstallation of distributor (V6 and V8 engines)

Turn the ignition key off, remove the negative side cable off the battery. Remove the components necessary to gain clear access to the distributor. Remove the connector at the distributor body.

Pull the distributor cap and look at the position of the rotor. The contact on the rotor should be facing the pointer on the distributor housing depending on the engine. There is a pointer for the V6 engine and one for the V8 engines. When the rotor contact is aligned with the appropriate pointer on the distributor, this is the position of the #1 plug in the firing order.  

The pointers are marked "6" for the V6 engine and "8" for the V8 engines. Turn the engine over if necessary to get the rotor contact aligned with the pointer. At the base of the distributor, place a paint mark on the base and the intake as a position indicator to properly position the distributor during reassembly.  Remove the hold down bolt and clamp and remove the distributor. Remove and replace the O-ring. 

If you are replacing the original distributor, look at  the position of the paint mark on the old one and use it as a reference to install the new one. 

When installing the distributor, turn the rotor contact counterclockwise-opposite of normal engine rotation roughly 42-45 degrees from the proper pointer on the distributor housing and then install the distributor into the block. When it engages the cam gear teeth the rotor will rotate clockwise to the proper pointer mark on the distributor, when it is fully seated.

The distributor must be installed in the same position as originally installed. The spark plug terminals on the distributor must be aligned perpendicular to the engine centerline and the rotor must remained aligned in its proper mark.

If the crankshaft has been moved while the distributor is out, the #1 piston must be on TDC before reinstalling the distributor. To reposition the #1 position, you need to pull the number 1 spark plug and rotate the engine clockwise while feeling for compression pressure. The best way is to have your finger over the spark plug hole while someone cranks the engine over, either by using a socket/wrench on the crankshaft pulley bolt or by bumping the starter until you start to feel pressure. Once you start to feel the compression pressure, stop, and then rotate the crankshaft slowly until the mark on the crankshaft damper is aligned with the TDC "0" degree mark on the timing indicator.  Then repeat the installation step noted above.

It is easy to see the relation between the alignment of the rotor contact, distributor pointer and the position of the #1 plug position (V8 shown, V6 similar).



This is the CKP or crankshaft position sensor. It s located in the front timing cover, and is activated by a reluctor  wheel mounted on the crankshaft. The 4.3L V6 has 3 slots, therefore is triggered 3X per revolution. The V8 engines are triggered 4 times per revolution. 



The "plastic" timing chain cover used on the early Vortec V6 and V8 engines.      The reluctor under the cover, that is keyed for proper timing on the crankshaft, V8 reluctor shown on the left, the 4.3L V6 is on the right.


This picture shows the relationship between the reluctor and the crank sensor. The V6 uses a 3x reluctor, the V8 uses a 4x reluctor. The crankshaft as you know, has to make two complete revolutions to complete the firing order. 


                 1992         4.3L V6 CPI      S-10 Truck, Astro Van             16197427
                 1993         4.3L V6 CPI       S-10 Truck, Astro Van             16197427
                 1994         4.3L V6 CPI       S-10 Truck, Astro Van             16197427
                 1994         4.3L V6 CPI            C/K Trucks, Van             16196395
                 1995          4.3L V6 CPI        S-10 Truck, Astro Van             16197427
                 1995         4.3L V6 CPI             C/K Trucks, Van     16196395, 16245358
                 1996         4.3L V6 CSFI       S-10 Truck, Astro Van              16208546
                 1996  4.3L V6 ; 5.0, 5.7L V8 CSFI          C/K Trucks, Van              16244210
                 1997  4.3L V6 ; 5.0, 5.7L V8 CSFI    S-10, C/K Truck, Van              16229684
                 1998         4.3L V6 CSFI    S-10 Truck, Astro Van     16258515,, 16250279
                 1998        5.0, 5.7L CSFI   V8          C/K Trucks, Van      16250279, 9355699
                 1999         4.3L V6 CSFI     S-10 Truck, Astro Van      16250279, 16263494
                 2000         4.3L V6 CSFI    S-10 Truck, Astro Van               16263494

This list is only partial, and may not accurately reflect all the applications available.